Various types of inductors, such as chokes and saturable core reactors, are used in electrical power generation, conversion and transmission applications. For example, chokes may be used to reduce the AC ripple in a rectified current or as components of EMI filters to remove high frequency interference, while saturable core reactors have been used in magnetic switches for voltage regulation. Conventional inductors for electrical power applications are usually formed by winding many turns of a conventional conductor, such as copper wire, either around an air core or around a central core of high permeability material, such as steel. Inductors which use either type of core may have significant disadvantages for electrical power applications which must carry high currents or sustain multi-kilowatt power loads. FIG. 1 shows a cutaway view of a conventional air core inductor, including multiples turns of conductor, 100. Since inductance is a function of the number of turns of the conductor and the permeability and geometry of the core material, air core inductors need to be extremely large and heavy and to use a great deal of wire to achieve large inductances. Not only is there an upper limit on the inductance obtainable in small geometries, but air core inductors have high leakage and stray flux which interfere with other parts of the system and defeat the goals of many electrical power system designs.
In consequence, a highly permeable core is employed in many conventional inductor designs to permit the desired inductance to be achieved with fewer turns of conductor and to constrain the flux to a controlled path within the core to limit stray leakage. However, the approach is not without consequences. Large and highly dissipative structures are needed to control flux density and core temperatures to prevent saturation of the core and to limit the operating temperature of the windings for a desired energy storage. The larger the physical size of the inductor, the larger the problem since the volumetric heat generation is a constant while the surface area of the inductor winding, from which heat is rejected, does not scale linearly with volume. Often, these cores are designed with a high reluctance air gap to prevent magnetic saturation of the entire inductor in the presence of a high DC bias with additional AC flux. The resulting reduction in permeability requires a compensatory increase in the number of turns of winding required to maintain the desired inductance, and increased conductor area is then required to reduce dissipative levels within the windings to acceptable levels. As an additional consequence of introducing an air gap, the gap length must be limited to control fringing and leakage flux. Distributed air gap cores, formed by compressing and sintering powdered iron or iron alloys, can be used to solve this problem, but limitations exist on the size of commercially available individual cores of this type due to the compressive forces required in the manufacturing process. In short, conventional inductor designs are inherently large, heavy and lossy at electrical power frequencies, particularly in designs intended to carry more than 50 amperes of current or to provide multi-kilowatt power capacity, and these designs must use extremely long lengths of conductor to obtain desired inductances. In power system designs based on magnetic linkages between multiple inductors, these problems are exacerbated.
The disadvantages of these conventional designs become even more pronounced when the use of high temperature superconductor windings is considered. High temperature, or oxide, superconductors have many desirable electrical and magnetic properties, of which the most notable for inductor applications is the near complete absence of losses when carrying DC currents. In addition, when cooled to a superconducting state, they are capable of carrying very high currents, with densities of thousands of times that of conventional copper conductor. Thus, they have gained increasing attention for having the potential to improve the efficiency of electric power and magnetics applications, particularly those involving currents with a large DC bias. AC losses do occur, varying with frequency, AC and DC amplitude and conductor geometry but they are much lower than those in conventional conductors. At 60 Hz and 77K, the effective resistance of a superconducting composite would be less than 1/10,000 of the effective resistance of a conventional copper conductor operated at ambient temperature. Oxide superconductors also have ability to maintain these properties at relatively high cryogenic temperatures, in the general range of the boiling point of nitrogen. In this temperature range,(unlike the much lower range of temperatures at which low temperature superconductors operate), coolants are relatively inexpensive and safe to handle, and FETs and other electronic devices may also be operated, making possible the design of complex power systems such as DC power supply systems including fault current limiters and filters. Thus, the use of oxide superconductors in conventional air core inductor designs has been proposed, particularly in cases where resistive losses needed to be minimized or the acoustic noise level associated with air gapped core designs was unacceptable.
However, the potential benefits of oxide superconductors are offset by some very significant physical limitations which limit their suitability for conventional inductor designs. High temperature superconductors are not metals, but fragile, brittle, ceramic-like compounds which cannot be drawn into traditional conductor forms in their natural state. Traditional conductor forms such as wires, tapes, and cables are generally made by forming composites of one or more high temperature superconductive filaments in combination with noble metals, such as silver, to form more ductile conductors. However, these composite conductors are still brittle by the standards of conventional conductors, and cannot be wound or otherwise bent to a tight radial arc without a reduction of available current density. High temperature superconducting composites would not stand up to the winding stresses of conventional inductor designs. Because of their high noble metal content, they are also quite expensive in comparison to conventional copper conductors, a very significant factor if large amounts of conductor are to be used in the design. In addition, the cost of cooling the entire winding to cryogenic temperatures must be factored in, and the large size of the structure adds significantly to the thermal loading of the necessary cryorefrigerator. A practical design for a superconducting inductor specifically adapted to the strengths and limitations of high temperature superconducting conductors has yet to be provided.
Thus, an object of this invention is to provide novel inductor designs that will allow a high temperature superconducting composite to provide the conducting portion of an inductor.
Another object of the invention is to provide improved inductor designs with extremely low resistive losses in comparison to conventional designs for high power, high current applications at power frequencies.
Another object of the invention is to provide relatively lightweight, compact inductor designs for high current or high power applications.
Another object of the invention is to provide inductor designs for high current applications with improved performance characteristics under transient conditions.